The present invention relates to optical fibre communication lines that are configured to operate in stand-by, that is to attenuate or substantially suppress the transmission of signals, but can still be monitored for cable ruptures or other defects. The invention is particularly relevant to optical systems wherein a redundancy of optical fibre cables is provided for added protection, and specifically to wavelength division multiplexed systems. The invention further concerns optical fibre amplifiers suitable for use in an optical communication line that may be configured to operate in standby, and a method for monitoring the operation of an optical communication line configured to operate in standby.
In many optical communication applications it is important to block the transmission of signals through individual paths, at least temporarily. One application is the duplication of an optical link connecting terminal stations so that operations can continue following failure of one of the lines. Such an arrangement is of particular importance for optical wavelength division multiplexed (WDM) systems in which large amounts of traffic are borne by a single fibre.
In a known WDM point-to-point system multiple signals are multiplexed at a transmitting terminal onto a single optical fibre cable which is then split into two paths using a fibre coupler. The signals on each path are amplified at the transmitting end by an optical fibre power amplifier and at the receiving end by an optical fibre pre-amplifier. Optical fibre amplifiers comprising a doped fibre into which is coupled pumped optical radiation from a pumping laser are well known in this field and are for example disclosed on pages 402 to 404 of IEEE Proceedings J, vol. 135, no. 6, 1988 pp 385-407, P. Urquhart, xe2x80x98Review of rare earth doped fibre lasers and amplifiersxe2x80x99. The two paths are then recombined using a fibre coupler at the receiving terminal prior to demultiplexing. Further optical amplifiers may be provided along the paths to compensate for signal attenuation if the distance between the terminal stations is large.
At any one time only one of these paths should be active to prevent interference cross talk between the signals at the receiver. However, if redundancy is to be preserved, the inactive line must be in working order. Thus it is desirable to continually monitor the state of the inactive or standby line so that any fault can be detected and if possible repaired.
In the known system this is done to a limited extent by keeping all but the final optical fibre amplifier active. The final amplifier, generally the pre-amplifier, is almost turned off thus blocking the signals. If the pumping energy is sufficiently low, substantially all light energy input into the amplifier will be absorbed.
Since the signals in the redundant path are propagated up to the pre-amplifier, the correct operation of the path upstream of this amplifier can be verified in the normal fashion by monitoring the propagated light signals.
However it is also desirable to monitor the fibre section between the pre-amplifier and the demultiplexer as well as the operation of the pre-amplifier itself. A possible solution is to have the pre-amplifier xe2x80x98glowingxe2x80x99 i.e. to drive the pump laser at a low power, so that a signal resulting from amplified spontaneous emission (ASE) generated in the amplifier is propagated by this fibre section. The power should be so low that the signal is attenuated sufficiently not to cause cross talk penalty at the receiver, but large enough to be monitored as light at the input of the receiver. The problem with this arrangement is that reflections of the light signal from the working section occurring at the various device interfaces may be stronger than the low power light from the xe2x80x98glowingxe2x80x99 pre-amplifier. As a result it is very difficult to determine whether light detected on the standby path downstream of the xe2x80x98glowingxe2x80x99 pre-amplifier originates from this pre-amplifier or from the working path. Accordingly a broken fibre in this fibre section may not be detected.
It is therefore an object of the invention to provide an optical communication line which may be monitored reliably and in its entirety even when configured to block signals.
In an optical communication line that may be configured to block signals using an optical fibre amplifier, this object is achieved according to the invention by providing modulating means adapted to superimpose a control tone on the amplifier output signal at least when the amplifier is configured to attenuate received signals, and by providing monitoring means downstream of the amplifier for detecting the control tone. In a preferred embodiment, the pumping radiation of the optical amplifier is modulated, resulting in modulated amplified spontaneous emission at the output of the amplifier.
By modulating the amplifier output signal with a control frequency, a recognisable and distinct signal will be generated even when the amplifier is driven to attenuate any received signals. Since the modulating means specifically detect this control frequency rather than simply a predetermined level of light energy, the correct functioning can be reliably determined simply by ascertaining the presence or absence of this control tone.
According to a further aspect of the invention this arrangement is applied to an optical communications system with transmitting means and receiving means that are interconnected by at least two optical fibre cables arranged in parallel. An optical fibre amplifier on one of the lines comprises modulating means for superimposing a control tone on the amplifier optical output signal at least when the amplifier is configured to attenuate signals. The control tone is detected by monitoring means arranged downstream of the amplifier. When this system is configured with one path working and the signals in the other path blocked by the optical fibre amplifier, the correct functioning of the inactive cable can be reliably ascertained. In particular, the risk of mistaking reflections from the working cable for the low power light from the optical amplifier of the standby cable is substantially eliminated, since the presence of the control tone frequency is the criteria for determining a functioning connection between the amplifier and the receiving means. In a further embodiment of the invention this arrangement is applied to a wavelength division multiplexed system.
A further aspect of the invention resides in an optical fibre amplifier suitable for use in an optical communications line which has modulating means adapted to superimpose a control tone on the amplifier output signal. This amplifier is capable of emitting a distinct and recognisable signal, even when operating at low power to block incoming light signals, and hence enables both its own function and the integrity of any subsequent optical fibre link to be verified reliably.
The above object is further achieved according to another aspect of the present invention in a method for monitoring the operation of an optical communication line including modulating the amplitude of the optical amplifier output signal a control tone and ascertaining the condition of the line by determining the intensity of said control tone on said optical fibre cable.